1. Field of the Invention
The invention relates to a laminated panel assembly comprising at least one rigid panel, an electrically controllable flat functional element, such as an electroluminescent lighting element, and at least one electrically conducting thin film.
2. Description of the Background Art
The term “laminated panel assembly” is understood here to mean a unit consisting of a rigid panel and of the functional element, which is itself composed of several films and joined to the rigid panel, or alternatively applied thereto. The term “functional elements” is understood to mean, within the context of the present description, flat elements such as electroluminescent lighting elements or also panel elements with electrically controllable light transmission, for example elements of the liquid-crystal type. The respective functional element therefore has optical properties that can be electrically controlled.
It is not absolutely necessary to place the functional element between two rigid panels of a laminated panel assembly, but this arrangement will be preferred for safety reasons, given that there is possibly a rather high supply voltage (for electroluminescent lighting elements). Placement in a laminated panel assembly furthermore protects the functional element from mechanical attack and from penetration by moisture and dirt.
The material of the rigid panels may themselves in principle be chosen freely; likewise, it is possible to use plastic panels or glass panels. It is also known that glass panel/plastic panel hybrid laminates can be manufactured on an industrial scale.
Document EP-A1-0 267 331 teaches a laminated panel assembly for vehicles with a sign placed in the adhesive layer of the laminate, which sign is represented or can be back-lit by an electroluminescent (EL) lighting element. The necessary electrical conductors are rendered practically invisible by using transparent conducting tracks or layers inside the laminate. After the supply voltage has been applied, the luminous signal seems to float in the panel assembly without the conductors being visible. The aforementioned document presents two different types of lighting elements. In the first type two conducting electrodes are provided on the same substrate, these being coated by the lighting element, which in turn includes a junction electrode. From the electrical standpoint, two capacitors connected in series are thus formed. In the second type, one of the two electrodes is respectively deposited in the form a thin transparent layer on the two internal surfaces of the laminated panel assembly, and the lighting element is placed between these surfaces. Also disclosed in that document, as an option, is the fact that the output of light through one of the panels may be prevented by means of an opaque coating.
The prior German patent application 10126868.6 discloses a panel with a ruled opaque coating, in which panel, at least in part of the opaque surface portions, at least one flat EL lighting element consists of several layers with a transparent electrode is provided, which lighting element, after an electrical voltage is applied, emits light on the side with the transparent electrode of one of the flat faces of the panel. A laminated panel assembly of this type can be used, for example, as a roof panel in a vehicle, which, when it is dark, illuminates the passenger compartment by internal surface lighting.
For most fields of application of laminated panel assemblies, it is desirable for the luminous tint from the EL lighting element to be practically constant. When such EL lighting elements, made of a thick inorganic film of large area, operate, especially in motor vehicles, it turns out that differences in the color emitted may, however, result when there are variations in temperature. Thus, an EL lighting element at −20° C. will shine with a completely different color than at +80° C., these temperature variations nevertheless being realistic during the use of a motor vehicle. A given luminous tint, whose color harmonizes, for example, with its surroundings, can only be set for a certain limited temperature range.
The abovementioned literature does not tackle this problem. Admittedly, these lighting elements in operation gives off a certain amount of heat because of the power that they dissipate; however, in practice this is negligible.
Trials carried out for the purpose of balancing the luminous tint by injection of the frequency of the supply voltage result in a drastic reduction in the lifetime of the EL lighting element.
It is certainly possible, by using a temperature sensor, to completely prevent the EL lighting element from lighting at low temperatures. Thus, negative effects of low-temperature operation on the lifetime of the EL lighting element are also avoided.